1. Field of the Invention
The present invention relates generally to a shield case for electromagnetically shielding a radio frequency or high frequency circuit, and, more particularly, to such shield case of the type having through-hole means in which connecting means such as a connector is to be mounted.
2. Description of the Related Art
A radio frequency circuit is used in electronic apparatus or devices and electronic instruments handling radio frequency or high frequency signals such as a spectrum analyzer for measuring distributions of the components of various frequencies of an input signal as well as a signal generator comprising a high-frequency power supply used for adjustment, testing and the like of radio receivers, high-frequency amplifiers or the like. Generally, the high frequency circuit of the type concerned is either formed in a printed circuit board or mounted in a printed circuit board as a semiconductor integrated circuit (IC). Typically, a printed circuit board on which a high frequency current exceeding 1 GHz flows is housed in a shield case in order to prevent spurious radiation from being emitted out from the printed circuit board.
FIG. 3 illustrates an example of the prior art shield case of the type described herein. The illustrated shield case 2 comprises an open-topped shield case body 2b which is typically of generally rectangular shape in a plan view approximately analogous to that of a printed circuit board 1 accommodated therein, and a shield cover 2a for closing the open top of the shield case body 2b. Once the printed circuit board 1 has been housed in the shield case, the shield cover 2a is secured to the open top of the shield case body 2b by fastening means such as screws to electromagnetically completely shield the interior of the case.
In this example, the printed circuit board 1 has a signal source 1a mounted thereon for generating various high frequency signals with the connection between the printed circuit board 1 and an external circuitry or equipment being made by feed-through capacitors 3 in the illustrated example. Specifically, the central conductors 31 of these capacitors 3 are used as feed-through terminals to connect the printed circuit board 1 with the external circuitry or equipment 32 to transmit and receive signals (mainly control signals) therebetween.
In modern technology, it may be required to connect as many transmission lines (wires or cables ) as several tens for control signals and other signals with the printed circuit board 1 enclosed in the shield case 2 since the radio frequency (RF) circuit requires numerous control signals for the various switches and digital-analog converters (D/A converters) used in that circuit. However, it is difficult due to the limited space to dispose so many feed-through capacitors 3 or feed-through terminals in the bottom wall or side walls of the shield case 2, and if it were feasible, it would involve a very high cost and would be objectionable in terms of cost.
In an attempt to overcome this problem, it has been proposed by the inventor as an alternative approach as illustrated in FIG. 4 to connect several tens of signal transmission lines 52 with the connector 5 and to form the bottom wall of the shield case 2 in this example with a through-hole 6 (which is usually in the form of a slit or slot as the connector of this type is thin in thickness) through which the connector 5 is to be inserted and plug the connector 5 in a connector socket 52 disposed in the printed circuit board 1.
However, it is to be appreciated that with such through-hole 6 for mounting the connector 5 formed through the shield case 2, that portion of the case formed with the hole 6 would not be electromagnetically shielded, so that the bottom wall of the shield case 2 would have an effectively open slit 6 having a length of a few cm. As a consequence, the shield case 2 would act as a slot antenna with a cavity resonator, resulting in intensifying a specific frequency and radiating it to the outside. That is, a spurious radiation with an enhanced level is emitted out through the slit 6 to the exterior of the shield case to deleteriously affect the surrounding equipment and circuits.
Inability to suppress such spurious radiation has heretofore made it impossible to connect the printed circuit board 1 accommodated in the interior of the shield case and having a radio frequency current higher than 1 GHz passing therethrough with the external equipment or circuitry by means of the connector 5 having a multiplicity of signal transmission lines connected therewith.
It is an object of the present invention to provide a shield case which is capable of significantly reducing the level of spurious radiation which may be emitted out through a through-hole formed in the shield case for mounting connecting means such as connector means therein.
According to the present invention, the foregoing object is accomplished by providing a shield case having a through-hole formed therethrough in which a material giving a large transmission loss to a radio frequency signal passing therethrough is applied to such a surrounding area portion of the interior surface of the shield case as surrounding the through-hole as well as to the inner peripheral surface of the through-hole.
In a preferred embodiment of the invention, the material giving a large transmission loss to a radio frequency signal applied to the surrounding area portion of the interior surface of the shield case surrounding the through-hole is contiguous with the material giving a large transmission loss to a radio frequency signal applied to the inner peripheral surface of the through-hole.
Preferably, the surrounding area portion of the interior surface of the shield case surrounding the through-hole is formed with a recess in which the material giving a large transmission loss to a radio frequency signal is embedded.
In a specific embodiment of the invention, the shield case 2 has accommodated therein a printed circuit board 1 having a radio frequency circuit 1a, a connector being inserted in the through-hole 6 for connection with a connector socket 52 disposed in the printed circuit board, the connector having a multiplicity of signal transmission lines 51 connected therewith for electrically connecting the printed circuit board 1 and an external equipment or circuitry 32. (See FIG. 1A).